Higher temperatures in automotive underhood applications and customer demands for wider service temperature ranges and longer warranty periods have led to the need for belt elastomers with improved high and low temperature resistance. As a result, ethylene-alpha-olefin polymers have replaced polychloroprene as the primary elastomer for automotive multi-V-ribbed belt applications.
A known disadvantage of ethylene-alpha-olefin elastomers, however, is inferior wear resistance. A particular problem inherent to this type of elastomer is the tendency to undergo chain scission in response to dynamic loading and abrasive wear. In the case of multi-v-ribbed belts, this characteristic leads to what is referred to as “pilling”, that is, the accumulation of sticky abraded material in the grooves of the belt. This phenomenon is a common source of underdesirable belt noise and, in extreme cases, can lead to catastrophic failure of the belt.
Several methods for improving the pilling resistance of ethylene-alpha-olefin belts are known and described in the prior art. One method for improving pilling resistance involves use of an ethylene-alpha-olefin elastomer with high ethylene content. One disadvantage of this approach is that high ethylene grades of these elastomers are difficult to process on the open mills and calenders typically used in the manufacture of power transmission belts. Ethylene-alpha-olefin elastomers with high ethylene content also exhibit poorer low temperature properties than comparable elastomers with low ethylene content due to high glass transition temperatures and a greater degree of crystallinity.
Another approach to improving pilling resistance of compounds based on ethylene-alpha-olefin elastomers is the incorporation of high tenacity and wear resistance fibers, particularly aramid fibers such as those sold under the trademarks Kevlar, Twaron and Technora. The use of such fibers to improve pilling resistance is described in a number of prior publications including Yarnell et al. (U.S. Pat. No. 5,610,217), Sedlacek (U.S. Pat. No. 4,798,566) and Takehara et al. (U.S. Pat. No. 6,177,202).
When the grooves of multi-V-ribbed belts incorporating such fibers are formed by cutting or grinding, a portion of these fibers are exposed and protrude from the rib surface. These fibers essentially function to minimize contact between the base elastomer and the belt pulley, rather than by enhancing the pilling resistance of the base elastomer itself. A major disadvantage to the use of such fibers, however, is their significantly higher cost relative to other fibers commonly used in power transmission belts. For example, the raw material cost for aramid fibers is typically more than 20 times the cost of suitable cotton fibers. In addition, aramide fibers are difficult to disperse effectively on typical rubber mixing equipment, thus requiring more extensive and higher cost mix procedures.
A third approach to overcoming the problem of pilling in multi-V-ribbed belts is to incorporate a material in the elastomer compound that will migrate to the surface of the belt rib and lubricate the contact portion between the belt and pulley. An example of this approach is the use of polysiloxane material described by Connell, et al. (U.S. Pat. No. 5,284,456). This method has several disadvantages, however. Reduced friction between the power transmission belt and pulley surfaces limits load carrying capability. Furthermore, the rate of migration of such materials is strongly dependent on environmental conditions, and the concentration on the belt surface is dependent both on environmental conditions and frequency of use. In automotive applications which require an extremely wide service temperature range and duty cycles, this leads to insufficient surface lubrication under some conditions and excess lubrication in others.
A critical property required of elastomeric compositions used in power transmission belting is the availability to transmit load under dynamic conditions. Compounds used in this application have therefore typically included high levels of reinforcing particulate fillers. Carbon black and silica are the two most commonly used reinforcing particulate fillers. These fillers, unfortunately, have several disadvantages when used in power transmission belts. On disadvantage is they cause a decrease in flex/fatigue resistance proportional to their level, due to increased strain energy input with each cyclic deformation. Another disadvantage of compounds with high levels of reinforcing particulate filler is an increase in compound hysteresis, which relates to the amount of energy dissipated by the compound under dynamic loading. High hysteresis compounds subject to dynamic loading exhibit greater heat buildup than compounds with lower hysteresis, accelerating the heat aging process and, therefore, leading to premature failure. High hysteresis compounds also exhibit poorer wear resistance, especially pilling, than comparable low hysteresis compounds.
The use of non-migratory internal lubricants in power transmission belts has been described, for example, by Brew (U.S. Pat. No. 4,464,153), Standley (U.S. Pat. No. 4,244,234) and Henderson et al. (U.S. Pat. No. 4,032,768). In all of these cases, the lubricant was used to reduce the co-efficient of friction between the belt and pulley. In many applications, particularly the multi-V-ribbed belts, co-efficient of friction needs to be kept above some minimum value corresponding to the design of the belt drive or customer requirements.
Due to the disadvantages associated with known methods of improving wear resistance and minimizing hysteresis in ethylene-alpha-olefin elastomers, while maintaining acceptable load carrying capability, there exists a need for an ethylene-alpha-olefin elastomer which will exhibit low hysteresis and good wear resistance, especially pilling, without requiring the use of high ethylene polymer, aramid fiber, migratory lubricants or high levels of reinforcing particulate filler. There further exists a need to meet these requirements without reducing the co-efficient of friction between the elastomer and standard belt pulleys.